1. Field of the Invention
This invention relates to an image reading lens and an image reading apparatus using the same and having a controller circuit for converting the image information of a reading element into an output signal, and particularly is suitable for an image scanner or a film scanner for reading the image information of an image by a reading element such as a line sensor (CCD), and a digital image reading apparatus such as a digital copier or a multifunction printer.
2. Related Background Art
An image scanner, a film scanner or the like is known as an apparatus for reading a paper image or image information on film as digital image information for a computer.
For an image reading lens used in such a scanner apparatus, it is desired that distortion and chromatic aberration be corrected well and that the reduction in the amount of marginal light by vignetting be small. Also, because of the use of reading the image information of an image, it is desired that usually lateral magnification be set within a relatively low magnification range of the order of xe2x88x920.1 to xe2x88x921.2 (about one-to-one magnification).
Further, in recent years, the tendency toward downsizing has advanced and the compactness of an optical system layout has been required and so, a wider angle has been required of a projection lens (image reading lens) which determines the optical system layout.
Various image reading lenses of this kind are proposed, for example, in Japanese Patent Application Laid-Open No. 11-190820, Japanese Patent Application Laid-Open No. 2000-241701, Japanese Patent Application Laid-Open No. 2000-249912, etc.
In the above-mentioned Japanese Patent Application Laid-Open No. 11-190820, there is proposed an image reading lens comprising a four-lens construction, but because of the small number, four lenses, if astigmatic difference is suppressed, curvature of field becomes great, and in accordance therewith, spherical aberration is corrected so as to keep balance from the on-axis to the off-axis, and this has led to the problem that such image reading lens is insufficient in performance for an image reading apparatus of high resolution.
In the image reading lens proposed in Japanese Patent Application Laid-Open No. 2000-241701, the number of lens components is increased to five to thereby suppress curvature of field relatively low.
The image reading lens proposed in Japanese Patent Application Laid-Open No. 2000-249912 uses an orthometa type of six-lens construction to thereby make curvature of field small and make the depth of focus great. However, this lens has suffered from the problem that changes in the curvatures of sagittal image field and meridional image field change in opposite directions at a medium angle of field and astigmatic difference occurs greatly and this lens is insufficient in performance for an image reading apparatus of high resolution. Also, the angle of field of this lens is narrow as compared with the lenses of the aforementioned publications, and this has led to the problem of being insufficient to achieve compactness.
In recent years, higher resolution for obtaining a higher quality of image has also come to be required.
It is an object of the present invention to provide a bright image reading lens of which the lens construction of each lens is appropriately set to thereby cope with high resolution and yet which has a great depth of focus and a wide angle of field, and an image reading apparatus using the same.
It is another object of the present invention to provide a bright image reading lens of six-unit six-lens construction having an angle of field as wide as a half angle of field of 23xc2x0 or greater and F number (Fno) of the order of 5, and an image reading apparatus using the same.
An image reading lens according to one aspect of the present invention has, in succession from the object side, six lenses, i.e., a meniscus-shaped positive first lens having its convex surface facing the object side, a second lens of which the both lens surfaces are concave surfaces, a third lens of which the both lens surfaces are convex surfaces, a meniscus-shaped fourth lens having its concave surface facing the object side, a meniscus-shaped negative fifth lens having its concave surface facing the object side, and a meniscus-shaped negative sixth lens having its concave surface facing the object side, and when the focal length of the ith lens as counted from the object side is defined as fi and the lens thickness of the fourth lens is defined as d8 and the radius of curvature of the image plane side lens surface of the second lens is defined as r4 and the combined focal length of the fifth lens and the sixth lens is defined as fn and the focal length of the entire system is defined as f, it satisfies the following conditions:
0.91 less than f5/f6 less than 2.78
xe2x88x920.27 less than d8/fn less than xe2x88x920.16
2.48 less than r4/f less than 10.75
In a further aspect of the present invention, the image reading lens has a stop between the second lens and the third lens.
In still a further aspect of the present invention, when the combined focal length of the first lens to the fourth lens is defined as fp and the radii of curvature of the object side and image plane side lens surfaces of the sixth lens are defined as r12 and r13, respectively, the image reading lens satisfies the following conditions:
xe2x88x920.95 less than fp/fn less than xe2x88x920.71
xe2x88x924.80 less than (r12+r13)/(r12xe2x88x92r13) less than xe2x88x922.75
2.26 less than f/f3 less than 2.88
An image reading apparatus according to another aspect of the present invention uses the above-described image reading lens to form the image information of an image on the surface of reading means.